Interpretive Summary: The sense of smell in insects utilizes odor receptors in the nose to detect chemicals present in the environment and to send messages to the brain allowing for behavioral responses to them. However, the exact roles of individual odor receptors may be difficult to ascertain. Here we describe approaches for determining the function of an odor receptor when present in the insect, and methodology to study the odor receptor outside of the insect. Using the different methodologies, the odor receptor for a human attractant odor for the yellowfever mosquito was shown to be of primary importance in the specificity of responses of the insect to the attractant. These methods will be used by entomologists and physiologists to determine the function of odor receptors present in sensory organs of insects and other animals.

Technical Abstract:
Odorant receptors (ORs) in insects are ligand-gated ion channels comprised of two subunits: a variable receptor and an obligatory co-receptor (Orco). This protein receptor complex of unknown stoichiometry interacts with an odor molecule leading to changes in permeability of the sensory dendrite, thus determining responses of the affected olfactory sensory neuron. Here we discuss both in vivo and ex vivo methods used to characterize responses of ORs to known ligands and closely related compounds. In vivo methods involved extracellular recordings of responses of neurons known to express the OR of interest; ex vivo experiments utilized heterologous expression of the receptor complex in Xenopus oocytes. Results of both in vivo and ex vivo methods were comparable and demonstrated that the OR alone was responsible for the observed specificity of the system.